The present invention relates to a transmitter provided with an advanced function adapted to switch between pre-defined operating modes corresponding to specific process conditions.
Generally, process transmitters are used to monitor and control industrial processes by measuring various characteristics of process materials used in the process. Typically, such monitored process materials are fluids or fluid mixtures in either a liquid or a gas phase. As used herein, the terms “fluid” and “process fluid” include both liquid and gas phase materials and mixtures of such materials.
One characteristic of process fluid that is frequently monitored is pressure. The pressure may be a differential pressure, or it may be a line, gauge, absolute or static pressure. In some installations, the measured pressure is used directly. In other configurations, the measured pressure is used to derive other process variables. For example, a differential pressure measured across a flow restriction within a pipe is related to the rate of fluid flow within the pipe. Similarly, a differential pressure measured between two vertical locations in a tank is related to the level of liquid contained in the tank.
Process transmitters are used to measure such process variables and to transmit the measured process variable to a remote location, such as a control room. A transmission can occur over various communication mediums such as, for example, a two-wire process control loop, a wireless communications link, and the like.
In installations where the process variable to be measured is pressure, pressure sensors are typically used within the process transmitters. The pressure sensors provide output signals related to applied pressure. The relationship between the output signal and the applied pressure is known to vary between pressure sensors. Generally, such variations are functions of the applied pressure and the temperature of the pressure sensor, and such variations are sometimes a function of a static pressure.
To improve the accuracy of measurements taken by the pressure sensors, each pressure sensor typically undergoes a characterization process during manufacture. The characterization process involves applying known pressures to the pressure sensor and measuring the output of the pressure sensor. Typically, the data is also taken at different temperatures. For example, a pressure sensor might be characterized by a pressure of 0 and 250 inches taken at 10 evenly spaced intervals of twenty-five inches, fifty inches, and so on. Multiple data sets can be taken at different temperatures. The data is then fit to a polynomial curve, for example, by using a least squares curve fitting technique. The coefficients of the polynomial are then stored in a memory of the transmitter and used to compensate subsequent pressure measurements taken by the pressure sensor. In general, the characterization information may be stored as polynomial coefficients or as characterization values in a look up table.
In pharmaceutical, biopharmaceutical, and food and beverage applications, the industrial system and its components must typically be sterilized prior to use, which means that from time to time the system must be flushed out with steam, for example. Additionally, in some installations, there are subsystems within the process that must be maintained within a range of temperatures that is narrower than the typical characterization range.
Since the sensors are typically characterized over a series of intervals and temperatures, the fit of the polynomial within the narrower range of temperatures may lead to “residual” temperature errors at specific temperatures throughout the operating range.